1. Field of the Invention
The present invention relates to binding teeth, a sheet processing device, an image forming apparatus, an image forming system, and a sheet binding method.
2. Description of the Related Art
There are known binding devices using no metal staples in terms of the recent trend of resource conservation and recycling efficiency. As these kinds of devices, for example, a sheet processing device using a method of binding a bundle of sheets by applying pressure to the bundle of sheets from above and below, for example, with rugged teeth (crimp binding) has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2014-168890 (Patent Literature 1) or Japanese Unexamined Patent Application Publication No. 2014-162106 (Patent Literature 2). Furthermore, there is already known a finisher equipped with both a binding unit using no metal staples (a staple-free binding unit) and a binding unit using metal staples (a staple-using binding unit).
Patent Literature 1 has disclosed, with the aim of selecting optimum binding teeth for each number of sheets to be bound and crimp-binding the bundle of sheets, a technology for a sheet processing device to hold two types of binding teeth that differ in height of ruggedness (tooth height) and switch between the binding teeth according to binding conditions, such as the number of sheets to be bound and the thickness of the sheets. Furthermore, Patent Literature 2 has disclosed, with the aim of reducing “twist” of a binding processing unit, a technology for binding teeth that the tooth width of around the ends thereof is shorter than that of around the center thereof and the tooth height of around the ends thereof is lower than that of around the center thereof.
However, in the technology disclosed in Patent Literature 1, the device holds two types of binding teeth, resulting in an increase in size. Furthermore, in the technology disclosed in Patent Literature 2, the binding force may decrease according to the number of sheets to be bound. Especially, in the technology disclosed in Patent Literature 1, the binding teeth differ in tooth height, and a gap between the binding teeth is wider at around the ends, and are not configured to engage with each other; therefore, although the binding teeth have the effect of reducing “twist”, they are not intended to secure sufficient binding force at low-height portions. In either case, when a bundle of sheets is bound by crimp binding, there is a problem that the binding force varies according to the number of sheets to be bound, and decreases when the number of sheets to be bound is close to the upper and lower limit of a bindable sheet number.